Hey Bart,
This is definitely a fair question, it's something I'm consistently questioning, and so, is totally open to change and input from you and other forum users.
But the idea behind closing the forum threads is part of our tracking process, in the past we were 'losing' threads that had gone unseen by our team and users weren't getting a response. So to resolve this what we do is use the discourse assign feature to add team members to a topic.
Then of course the problem is that we can't leave everything assigned forever because our inboxes will become completely unmanageable. So we close threads once there has been a resolution or it becomes inactive. This means we can combine those to indicators of assigned & closed to track threads that need attention here: https://community.prismic.io/latest?assigned=nobody&status=open
So the idea is that we give users 7 days to reply to the last response of an unresolved thread before closing and letting everyone know to contact us to re-open.
For resolved threads where a post has been marked as 'Solution' the discourse 'solved' plugin automatically closes thread 1 day after the last reply. This can be changed, and again, this is open to suggestions of what this limit should be.
I agree and worry that the closing of threads can stifle conversation and the reopening process definitely isn't perfect, especially for first time users, but it's the best process I can find at the minute to make sure every thread is seen and taken care of.
Looking forward to your thoughts on this.
Thanks.